The role of HTLV-1 in mediating these diseases is related to the ability of the virus to evoke lymphocyte activation. This complex retrovirus encodes typical gag, pol and env gene products as well as unique regulatory and accessory genes encoded in pX ORF I-IV between the env gene and the 3' LTR. Emerging evidence indicates a critical role of the gene products encoded in pX ORF I and II in viral replication. Four proteins designated p12-I, p27-I, p13-II and p30-II are expressed from these highly conserved ORFs. p12-I is a hydrophobic protein that localizes in the endoplasmic reticulum and has four minimal SH3 bindinq motifs. Using molecular clones of HTLV-1 with selective mutation of ORFs I and II, we were the first to identify functional roles of p12-II and p13-II/p30-II in establishment of infection in vivo. Subsequently, we demonstrated a vital role on p12-I in viral infectivity in quiescent lymphocytes and in enhancement of NFAT-mediated transcription. Thus, we are the first to demonstrate an essential role for p12-I in early T-cell activation. We hypothesize that p12-I serves a critical rote in viral replication during early stages of the virus infection by enhance virus expression. As a result of these studies, our laboratory is in a unique position to test mechanisms by which these "accessory" proteins influence HTLV-1 replication and cellular gene expression. Specific aims addressed in our proposed research include: 1) Determine structural motifs of p12-I important in infection of quiescent T-cells and their influence in NFAT-mediated transcription; 2) Characterize the interaction of p30-II with the co-activators, p300/CBP, and test the role of acetylation in p30-II mediated transcriptional activity and the influence of p30-II in apoptosis; 3) Test the effects of HTLV-1 p12-I, p30-II, and Rex in virus expression in vivo by infecting rabbits with molecular clones with selective mutations in p12-I and p30-II -encoding pX gene regions. Our long-term goal is to understand the rote of p12-I, p30-II, and Rex in the establishment of HTL V-1 infection in vivo.